# Photon energies relative to movement, and relativistic mass Q's.

• Mzachman
In summary, the conversation discusses the concept of a photon having a rest mass of zero and the effects of the Doppler shift on its energy. It also delves into the idea of relativistic mass and how it relates to the size and density of an object. The main conclusion is that energy and mass are relative concepts, even in classical mechanics, and that the Doppler effect and length contraction can greatly affect the energy and size of a moving object.

#### Mzachman

Not sure if this should go here or in the relativity forum... let me know if I need to move it.

I was reading something, and it said this:

"A photon also has a rest mass, which is zero, even though a photon is always moving at the speed of light, and so is never at rest. But if an observer moves fast enough in the same direction as a photon, sees the photon as having less and less energy. By chasing the photon, it can be made to have as little mass-energy as you like. As you chase it faster and faster, the photon looks redder and redder, by doppler shift, and the energy of a very long-wavelength photon approaches zero as you approach the speed of light."

Now, in my limited knowledge of relativity, light is always supposed to go the same speed, no matter what speed you are traveling, right? Are they trying to say something different here, or are they just saying that because the energy is related to the wavelength, the dopler effect can basically make the wavelength go to infinity and cause the energy to go to 0?

If that is true, doesn't that present and interesting problem of the energy of light/photons being completely relative, even if they are still moving (a moving photon particle still moving at the speed of light would have effectively 0 energy, which doesn't make sense to me)?

I could be completely wrong here, but it was just something I was wondering.
My other question is a little quicker. As something goes faster and faster, therefore gaining mass according to relativity, does the object become more dense? Bigger? etc... I have a thought about that, but if there is a concrete answer for it already, I would like to hear that.Thanks for clarifying.

Your beginning doesn't make sense. If you are traveling in the same direction as the photon ahead of you, you can't see it, since it is going away from you. You can see photons in the same direction coming from behind, which will be red shifted.

Well, I don't think they mean literally "see" it. Either way, that whole paragraph is just confusing...

They probably mean that the Doppler effect can make the wavelength arbitrarily long and frequency arbitrarily low. Energy of photons (or anything) is already relative. Even in Newtonian mechanics, energy depends on reference frame. KE of a massive particle already depends on velocity, which is THE archetypal relative measurement. PE of a particle even depends on where you put the *origin* of your reference frame, never mind its speed. And this is all without reference to anything Einsteinian.

It's deceptive to talk about "Relativistic Mass," especially of a freely moving particle. It's not the inertia of the particle, nor anything having to do with Gmm/r^2. The object does not become bigger. It gets smaller due to length contraction. So its density increases.

## 1. What is the relationship between photon energies and movement?

Photon energies are directly proportional to the movement of the photon. This means that as the photon moves faster, its energy increases. This is known as the photon's relativistic energy.

## 2. How are photon energies and relativistic mass related?

Relativistic mass is a concept in physics that takes into account the increase in mass of an object as it approaches the speed of light. Photon energies are directly related to relativistic mass, as the energy of a photon can be thought of as its mass multiplied by the speed of light squared (E=mc^2).

## 3. What is the equation for calculating relativistic mass Q?

The equation for calculating relativistic mass Q is Q = m_0 * (1 + v^2/c^2)^1/2, where m_0 is the rest mass of the object, v is its velocity, and c is the speed of light.

## 4. How does relativistic mass affect the movement of an object?

As an object's velocity approaches the speed of light, its relativistic mass increases significantly. This increase in mass results in a greater amount of energy required to accelerate the object, making it more difficult to increase its speed.

## 5. What is the significance of studying photon energies and relativistic mass?

Understanding the relationship between photon energies and relativistic mass is crucial in many areas of physics, including particle physics and astrophysics. It allows scientists to accurately predict the behavior of particles moving at high speeds and provides insight into the fundamental laws of the universe.